In measuring a person's eyes for purposes of corrective lenses, a number of measurements must be taken. For purposes of taking such measurements, numerous specialized apparatus have been developed, and the various apparatus are often quite expensive. As a result, the length of time required for examination is necessarily increased when the examiner is forced to keep switching between measurement instruments. Therefore, an instrument which can perform several functions in a novel manner is desirable both from a capital investment point of view and from a time savings standpoint.
One particular area of concern in eye measurement is that of the lens blank size necessary for mounting in a selected frame, which lens blank is capable of being ground to a prescription correcting the patient's eyesight. The reason for this concern is that as the diameter of a lens blank is increased, the cost of the same likewise increases. This situation is further complicated where the pupil of a patient's eye is not necessarily centered in the frame. A simple measurement of the diameter of the frame opening is not sufficient to yield the lens blank size because the optical center, at the center of a circular lens, must be positioned directly in front of the pupil of the eye. Hence, if the pupil is offset from the center of the frame opening, so must the optical center of the lens blank be positioned away from the center of the frame opening. Therefore, the lens blank necessary to fill the entire frame opening while maintaining its optical center in this offset relation must be quite a bit larger than the dimensions of the frame opening itself. This situation can lead to errors in calculation of the lens blank size which in turn can cause the eye examiner to estimate the cost of the lens incorrectly to the patient.